1. Field of the Invention
The present invention relates to a magnetic recording medium having a coating type magnetic recording layer wherein ferromagnetic particles are perpendicularly oriented. More particularly, the present invention relates to a magnetic recording medium wherein the magnetic recording layer contains a hexagonal system plate crystal (hexagonal plate-like) magnetic powder, and the back coating layer is composed of a certain specific binder and has an electric resistance within a specific range.
2. Description of the Prior Art
Magnetic recording media are now widely used in the fields of audio and video recording, computers, magnetic discs, etc., and they are expected to be used in the fields of video floppies, etc. Accordingly, the amount of information to be recorded on magnetic recording media increases year after year, and there is an increasing demand for the improvement of the recording density of magnetic recording media.
With respect to the magnetic recording media such as magnetic tapes, it has been common to improve the magnetic characteristics by e.g. orienting acicular magnetic particles in a magnetic recording layer in a longitudinal direction. However, those with acicular magnetic particles oriented in a longitudinal direction are not suitable for a high density recording although they are capable of obtaining a high output in a low frequency region.
For this reason, it has been recently proposed to use a magnetic recording medium wherein a plate crystal barium ferrite magnetic powder having an easily magnetizable axis in a perpendicular direction, is used for a magnetic recording layer (Japanese Unexamined Patent Publication No. 195328/1982).
However, such a medium wherein the barium ferrite magnetic powder is used, has a difficulty that erase characteristics are inferior although it is excellent in the short wavelength recording characteristics. Besides, barium ferrite has by itself an electric resistance of as high as at least 10.sup.16 .OMEGA./cm.sup.2, and accordingly, in the case where no conductive substance such as carbon black is used in combination with barium ferrite, the magnetic recording medium is likely to adhere to the head, or is likely to adhere to a guide roller, a calender roller, etc. during the production process such as a coating process, and in an extreme case, an electric discharge noize will be generated. Therefore, it is conceivable to use a conductive substance in combination with the megnetic powder, and it is common to incorporate carbon black to reduce the electric resistance. However, as mentioned above, the electric resistance of the magnetic powder is so high that no adequate reduction of the electric resistance has been obtained by the incorporation of ordinary carbon black although it was thereby possible to reduce the electric resistance to some extent.
There is a restriction as to the amount of carbon black to be incorporated. If the amount of carbon black is excessive, magnetic characteristics, electromagnetic conversion characteristics, surface roughness, dispersibility are likely to be impaired.
In these days, magnetic discs are widely used in the fields of computers and magnetic cameras. Accordingly, the amount of information to be recorded on such magnetic discs increases year after year. As a result, there is an increasing demand for magnetic recording discs having high density recording capability.
The higher the recording density becomes, the more pronounced the problem of dropouts becomes. Namely, in the current recording system wherein a magnetic head is utilized, the spacing loss between the disc or tape and the magnetic head may be represented by 54.6 d/.lambda.[dB] where d represents a distance between the disc and the magnetic head, and .lambda. represents the recording wavelength. It should be understood from this formula that in a short wavelength recording with a high recording density, the rate of reduction in the output due to spacing is significantly greater than that of a longer wavelength. Accordingly, even a small foreign matter existing on the surface of the disc or tape is inevitably detected as a dropout.
As possible causes for the dropouts, there may be mentioned the falling-off of magnetic powder from the surface of the magnetic disc coated with such powder material, which results from deterioration of the coated film due to repeated applications of stress, or the scraping-off of the supporting member during the disc running, and consequential electrostatic adhesion of the powder, dusts or the like on the surface of the supporting member, and their transfer from the supporting member onto the coated magnetic layer surface. In order to prevent these undesirable phenomena, there have been proposed various methods, for example, against the former cause, some proposals have been made to improve the toughness of the coated layer, and against the latter cause, a coating composition of carbon black, graphite or the like kneaded with an organic binder or an antistatic agent, is coated on the surface of the supporting member opposite to the magnetic surface of the magnetic disc or tape (i.e., the rear surface), thereby reducing the electrostatic phenomenon on the supporting member, or a coating composition of e.g. silicon oxide kneaded with an organic binder is coated on the surface of the supporting member in an attempt to render the supporting member to be more tenacious and thereby to reduce the abrasion of the base material.
While these treatments are effective to suppress the tendency for an increase in the dropouts by the repeated running of the discs or tapes to a remarkable extent, the level of the dropouts cannot still be said to be adequately low under the existing circumstances, and it is necessary to further reduce the dropouts.